Do you know your PD? For glasses wearers, knowing your pupillary distance (or PD) is very important. Your PD is the distance between the center of each pupil and it serves an important purpose. With a correct PD, you can ensure that your lenses will be correctly positioned in their frame. At 39DollarGlasses.com we offer two informative ways to measure your pupillary distance. First, we recommend using our PD ruler to get an accurate measurement. Find the PD ruler here. Next, watch our video tutorial to get a step-by-step visual guide on measuring your pupillary distance!